In order to determine where in the sequence of events following antigen binding to the antigen-binding B cell the regulatory influences of T cells and macriphages are exerted, these events are being studied in the specific sheep erythrocyte-binding ABC populaion of mouse spleen. Early events that appear to follow antigen contact with B-ABC include (in approximate sequence): receptor redistribution (capping), accelerated receptor replacement, accelerated phospholipid synthesis after enzymatic stripping, T and B 0ABC proliferation, and surface isotype changes such as the loss of sIgD and gain of sIgG. We are attempting to demonstrate PFC precursor frequency changes in the ABC population also. These processes are being studied after in vivo immunization. (using gradient purification of ABC), and also in a primary culture system where more precise kinetics and control over the presence of various regulatory subpopulations can be obtained. TNP-binding B cells from TNP tolerant animals are also being examined to determine which of these early events occur in that population.